Bipolar Plankton
Our planet has now had cold polar regions separated by warm tropical seas for some 8–16 million years. This zone of warmer water would seem to present a barrier to creatures adapted to the polar oceans, dividing them into Arctic and Antarctic species assemblages. Indeed it is true that polar bears are only found in the Arctic and penguins in the Antarctic. However, other groups of organisms show a curious bipolar congruence, and occur in both polar seas. Is the bipolar distribution of these species a consequence of the free mixing of genes 8–16 million years ago? Or have these organisms pierced the barrier of the warmer waters girdling the planet's waist?
Researchers have taken a close look1,2 at the DNA of some bipolar species of foraminiferans, a group of microscopic plankton with calcareous shells (Fig. 1). The results were telling; identical DNA sequences are shared by individuals that inhabit both the southern and northern oceans. The amount of genes shared, and comparison with foraminiferan fossils in ocean sediments indicates that mixing between the two populations has occurred within the last 200,000 years. Mixing may have been periodical during this time span, or have occurred regularly. In fact, mixing might still be occurring!
Figure 1: A wide variety of Foraminifera shells are preserved in sedimentary rocks.
How these organisms, that can only survive in cold temperatures, have crossed the equatorial ‘spas' is not yet known. During the last ice age, water temperatures at the equator were much cooler, perhaps facilitating the global travel of polar foraminifers, as well as other cold-loving marine creatures. Cool currents off the coast of west Africa bring cooler temperatures closer to the equator, as do seasonal upwellings of cooler water in the subtropical zone. Temperatures are also colder in tropical oceans at greater depths. While these pockets of cooler water may create paths of entry for vagabond plankton, how these tiny, polar organisms manage to transverse the wide expanse of tropical oceans is still a mystery |
